DESCRIPTION (Applicant's abstract): Previously, we have demonstrated that the carotid baroreflex control of blood pressure during dynamic exercise is classically reset to the prevailing blood pressure of the dynamic exercise. Aortic baroreflex control of heart rate was also reset to the prevailing blood pressure without a change in maximal gain. Additionally, our findings suggest that at rest the arterial baroreflex function is modulated by aerobic fitness, cardiopulmonary baroreceptor load and increases in the intramuscular pressure. However, the mechanisms by which 1) the arterial baroreflexes are reset during exercise, 2) the interaction between increases in muscle pressure and the arterial baroreflex function, and 3) the effect of aerobic fitness on arterial and cardiopulmonary baroreflex function remain to the defined. We hypothesize that the feed forward mechanism of central command resets the baroreflexes in parallel with the induction of motor activity and continuous modulation of this resetting is provided by activation of intramuscular mechanoreceptors within the active tissue. We further hypothesize that increases in aerobic fitness results in an increased blood volume and a resetting of the aortic and cardiopulmonary baroreflex. In order to test these hypotheses we will use a historically unique experimental paradigm in which muscle tendon vibration will be used to selectively increase or decrease central command. During steady state exercise (a constant exercise pressor reflex input) with and without muscle tendon vibration, carotid arterial baroreflex function will be determined using procedures developed in our laboratory. Using a similar experimental paradigm and increasing intramuscular pressure by using lower body positive pressure, the interactions between central command and intramuscular pressure will be examined. Additionally, using techniques developed in our laboratory and measures of MSNA, NE spill-over and leg vascular conductance we will selectively model the aortic and cardiopulmonary baroreflexes of high and low fit endurance exercise trained subjects. A unique aspect of this investigation is that the data obtained is immediately relevant to the healthy human and will provide significant fundamental information for the investigation of blood pressure regulation during physical activity of patients with heart failure, hypertension and intermittent claudication.